It does not matter if the area is 2x2, 20x20, or 200x200, if one grows the same plants in the same location year after year after year, one will see a steady decline in production and quality and will see an accompanying steady increase of fungus and insect problems.
On a plot so small, it is impossible to infect one part of it with pests, fungal or insect. & to grow far enough away from them that they cannot follow.
So the build up will happen anyway.
If you grow brassicas you may be unfortunate enough to get clubroot.
These spores can lay dormant in the soil for 20 years, and one good downpour can move them all over a 20x20 plot very easily.
That's if you don't move them about on your shoes yourself.
So! Grow your brassicas in the same spot each year, but start them in 3" pots, putting them in deep when they have 4 true leaves, with a dessertspoon of lime in the planting hole.
Then it doesn't matter if you have clubroot, as the lime will raise the pH of the soil surrounding the roots, to a level where clubroot spores can't open.
Nor will you protect yourself from diseases like verticillium or fusarium wilt, as insects act as the vector from plant to plant over a wide area; that's why many professional growers use grafted plants, with rootstocks vigorous enough to overcome the infections.
Onions: Robinsons is a company in the UK known for providing seed of show quality & they have been growing in the same patches for over 70 years. Grow them only from seed, then you do not put any white-rot infected roots into the soil. Once it's in the soil, you'll not be able to grow onions or garlic in your plot again.
Which pests are you going to protect your plants from, in a 20x20 bed, by using rotation?
Which pests can't move, or be moved, the length & breadth of a plot that size?